1. Field of the Invention
The present invention according to one aspect relates to a sliding member and, more specifically, a sliding member employing sintered aluminum alloy which can reduce seizure and adhesive wear experienced when sintered aluminum alloy members are friction-slid with each other in oil. Especially, the present invention relates to a sliding member applied to a gear rotor set including an inscribed gear used for an oil pump for engine lubricant, for automatic transmission (AT) for fuel supply, a sliding member employing sintered aluminum alloy which can reduce seizure and adhesive wear experienced when a sintered aluminum alloy member and ingot metallurgy aluminum alloy member are friction-slid, or to a friction member applied to hydraulic control valve or an oil control valve used in a hydraulic cycling path of ATF (Automatic Transmission Fluid) in Automatic Transmission (AT), Manual Transmission (MT), power steering or the like.
According to another aspect, the present invention relates to an oil pump and, more particularly, to an oil pump having a gear rotor set formed of aluminum alloy, of inscribed gear type, for engine lubricant, automatic transmission (hereinafter simply referred to as "AT") or for fuel supply.
2. Description of the Background Art
As a first prior art example, a gear rotor set used in an oil pump for engine lubricant, AT or fuel supply will be described. The gear rotor set includes an outer rotor and an inner rotor set in the outer rotor. The gear rotor set is set in a pump case. Teeth based on any of trochoid curve, involute curve and hypo-cycloid curve is formed on an inner side of the outer rotor and on an outer side of the inner rotor.
Conventionally, the pump case has been formed of cast iron, while the gear rotor set in the pump case has been formed of iron based sintered body. Recently, in view of weight reduction, application of aluminum alloy has been studied. In the pump, lubricant oil at a temperature of 150.degree. C. or higher circulates, and therefore the temperature of the pump as a whole increases. At this time, if coefficient of thermal expansion of the material of the pump case is much different from the coefficient of thermal expansion of the material of the gear rotor set, clearance therebetween increases, reducing volume efficiency of the pump. Accordingly, when aluminum alloy is applied to the pump case, it is necessary to apply aluminum alloy to the gear rotor set as well, so that the pump case and the gear rotor set come to have coefficients of thermal expansion close to each other.
Japanese Patent Laying-Open Nos. 60-128983, 2-169881 and 4-99204 disclose a gear rotor set to which sintered aluminum alloy is applied, as well as manufacturing method thereof.
As a second prior art example, an oil pressure-flow rate control valve formed of aluminum alloy used in an oil pump for AT, MT or power steering will be described. The valve includes a valve case, and a valve spool arranged in the valve case. The valve spool has a substantially columner shape, with a groove formed in circumferential direction thereof. The valve spool reciprocates and slides in axial direction of the valve spool, with a lubricant oil interposed with the valve case. This reciprocating sliding controls oil pressure and flow rate of oil.
Conventionally, the valve spool has been formed of iron based sintered material or steel product, while the valve case has been formed of cast iron. Recently, in view of weight reduction, aluminum alloy has come to be applied to the valve spool and the valve case. However, different from the valve spool and the valve case formed of iron based material, the valve spool and the valve case formed of aluminum alloy are more susceptible to the problems of abrasive wear, seizure and sticking caused by friction sliding of aluminum alloy members with each other.
In order to solve such problems, Japanese Patent Laying-Open Nos. 60-209609 and 2-173472 proposed anodizing plating, Tuframe process or hard coating such as Ni--P plating of a sliding surface of the valve spool and valve case, or the surface of aluminum alloy used therefor.
The anodizing plating refers to a process of forming a film by anodic oxidation of aluminum. Tuframe process refers to impregnation of polytetrafluoroethylene (trade name Teflon) in addition to anodizing plating. Ni--P plating is implemented by plating the surface of aluminum alloy with N--P plating liquid.
However, the gear rotor set for the oil pump in accordance with the first prior art example suffers from the following problem.
Japanese Patent Laying-Open No. 60-128983 proposes a method of forming the inner and outer rotors using sintered aluminum alloy. Here, the sintered aluminum alloy refers to an alloy prepared by press compacting aluminum alloy powder having a prescribed composition, and heating and sintering the same. Further, it also refers to the sintered aluminum alloy subjected to recompression of sizing or coining. Accordingly, it is distinguished from aluminum alloy of which relative density is made as close as possible to 100% by plastic deformation such as hot forging or hot extrusion.
The gear rotor set formed of such sintered aluminum alloy may be applicable to a pump to which a relatively low torque is applied, such as an engine lubricant pump. In case of an AT oil pump, however, the inner rotor is rotated by a driving shaft and, at that time, a high stress from the shaft is exerted on the inner diameter portion of the inner rotor which is in contact with the shaft. Therefore, high strength and high hardness are required of the inner rotor to reduce wear and damage. Especially, the outer rotor is prone to seizure and wear with a mating material at an outer peripheral surface which slides over the pump case and at the tip of the teeth sliding over the inner rotor. Therefore, superior seizure resistance is required of the outer rotor.
Accordingly, it is difficult to apply sintered aluminum alloy having low strength to the inner rotor to which high torque is applied or to the outer rotor of which superior seizure resistance is required.
The aluminum alloy powder prepared by hot forging or hot extrusion has high strength. Therefore, the aluminum alloy powder can well be applied to the gear rotor set of AT oil pump described above in view of mechanical strength. Japanese Patent Laying-Open Nos. 2-169881 and 4-99204 propose manufacturing method thereof.
It is very difficult, however, to form the inner rotor or the outer rotor having sufficiently high dimensional precision using the sintered aluminum alloy fabricated by hot forging or hot extrusion. Therefore, the teeth of the inner rotor and the outer rotor must be all formed by machining including cutting, grinding, end mill, electrical discharge machining and polishing. Such process increases manufacturing cost of the inner rotor and the outer rotor, presenting economical problem.
In view of the problems described above, the inventors of the present invention proposed, in Japanese Patent Application No. 9-157137 entitled "Gear Rotor Set Formed of Powder Aluminum Alloy and Manufacturing Method Thereof", a gear rotor set formed of an aluminum alloy in which sintered aluminum alloy having appropriate porosity is applied to the outer rotor which requires superior seizure resistance, so that the seizure resistance is improved by the oil retaining effect of the pores.
For the inner rotor which requires high hardness and stiffness, powder forged aluminum alloy having high strength prepared by hot forging or hot extrusion is used, and therefore the problem of seizure or sticking can be prevented.
In any approach, however, the inner rotor is set in the pump case, and fit in and rotated by the rotary driving shaft formed of steel. In order to facilitate insertion of the driving shaft through the inner rotor, there is provided a clearance between the inner surface of the inner rotor and the contact surface of the driving shaft. When the driving shaft rotates, the inner surface of the inner rotor is hit by the driving shaft as there is a clearance. Consequently, the inner circumferential surface of the inner rotor is wore and damaged, or suffers from laminer peeling at the surface portion, caused by bearing fatigue resulting from difference in mechanical characteristics including strength, hardness and stiffness between the steel and the aluminum alloy.
The valve in accordance with the second prior art example suffers from the following problem. In the valve spool and the valve case to which aluminum alloy coated with hard coating is applied, the cost is higher as compared with the conventional ones formed of iron based material, causing economical problem. Further, peeling and friction of hard coating are observed when the valve spool slides.
A valve having a combination of a valve spool formed of aluminum alloy subjected to anodizing plating and a valve case formed of iron based alloy, or a valve having the combination of a valve spool formed of an iron based alloy and the valve case formed of an aluminum alloy subjected to anodizing plating suffers from the following problem, because of the difference in coefficient of thermal expansion between the valve spool and the valve case.
More specifically, when the temperature is low, clearance between the valve spool and the valve case is small, causing sticking where the valve spool cannot slide satisfactory. When the temperature is as high as about 150 to about 180.degree. C., the clearance is larger than an appropriate value, so that oil pressure is decreased, or flow rate of oil increases or fluctuates, degrading performance of the oil pump.
In order to solve such a problem, it is effective to make coefficients of thermal expansion of the valve spool and the valve case as close to each other as possible. For this purpose also, it is preferred to apply aluminum alloy both to the valve spool and the valve case. Japanese Patent Laying-Open No. 2-173472 proposes a method of ensuring high performance by defining difference in coefficients of thermal expansion of the valve spool and the valve case.
In this case, however, in order to avoid sticking and friction between the valve spool and the valve case formed of aluminum alloy, both are subjected to anodizing plating or resin impregnated Ni--P plating. Such process increases cost, while lowering reliability.